The olfactory system discriminates a vast number of extremely variable odor molecules and thus poses a complex problem in perception which can be split into two parts: first, what is the role of specificity in the interaction of odors and their receptors in determining olfactory discrimination; and second, how does the brain integrate information from the periphery to allow the exquisite discrimination of the olfactory system? Another level of complexity is added if one considers that olfactory neurons are regenerated constantly throughout the life of the animal. Thus the olfactory system allows the study of neural regeneration in an adult animal. The recent identification of Axel's (sponsor) laboratory, of a multigene family likely to encode individual olfactory receptors allows us to study some of the fundamental questions of olfaction. In this proposal we describe a series of experiments using the olfactory receptors as molecular probes to study olfaction. We exploit the functional and anatomical simplicity of the catfish olfactory system. Since fish smell a smaller number of odorants and are therefore likely to have far fewer receptors than mammals, we should be able isolate and characterize the entire catfish repertoire of receptors. To this end, we have already identified 20 catfish receptors. We will first characterize the ligand-specificity of cloned olfactory receptors in heterologous cell expression systems. Then, we will examine the patterns of receptor expression and neural connections in the olfactory system. The cloned receptors will serve as molecular markers of distinct neurons for in situ hybridization, immunohistochemistry and dye tracing analysis of the olfactory epithelium and its projections to the olfactory bulb. Data from these experiments should allow us to formulate models delineating the roles of receptor-ligand specificity and neural connections in olfactory discrimination. We will then test these models by perturbing olfactory receptor expression in transgenic animals. Thus studies of olfaction will allow us to address important problems concerning processing of information by the brain and neural regeneration.